In figure 2 an oblique view of the apparatus is shown. The washer frame has been raised and the baseplate has been moved to the side for visual clarity. The silicone washer is held in a recessed hole which has been counter-bored in the bottom of the washer frame. The probe carrier box is shown unlatched and partially raised on its hinge. In figures 3 and 4, the sample is shown in place with the washer frame lowered and the washer pressed firmly against the sample. 2.2. Operation In operation, the baseplate with the sample mounted on it is slid under the washer frame and the two horizontal-axis micromanipulators are adjusted to make the washer concentric with the edge of the electrode on the sample. Both the washer and washer frame are transparent allowing this adjustment to be made quickly and easily. During this adjustment, the probe carrier box is swung up as far as it can go to allow the operator maximum visibility; this is illustrated in figure 3. The verticalaxis micromanipulator is then used to move the washer frame down, pressing the washer against the sample. It is important that the washer be pressed against the sample hard enough to fill any air spaces or voids at the sample surface. This pressure extrudes the washer slightly as can be seen in figures 3 and 4. The probe carrier box is then swung down and latched in place. The probe can then be lowered through the concentric holes in the washer and frame to make contact with the electrode. This is accomplished by turning either of the two cam-shaft knobs, thereby rotating the cam against the probe carrier and forcing it down. The probe has a springloaded retractable tip to provide a reliable contact to the evaporated electrode on the sample without damaging it. 3. EXPERIMENTAL RESULTS The use of the technique described in the preceding section has virtually eliminated dielectric breakdown during MIS C (V) measurements at appliedbias voltages up to 12 kV across 150-um-thick sapphire wafers. In addition, further tests were carried out at higher voltages. A 125-um-thick sapphire wafer was tested to breakdown at 16 kV. One 350-μm-thick sapphire wafer was tested without breakdown up to 30 kV which was the approximate limit of our test capability. In summary, a new, simple, easy-to-use technique for suppressing premature dielectric breakdown during high voltage C(V) measurements has been described. The technique allows the use of a much larger applied-bias voltage than was previously possible. ACKNOWLEDGMENT I am indebted to J. D. Morgan for his contribution to the design and construction of some of the essential hardware, to N. Capone, Jr. for the drafting in the appendix of this report, and to J. M. Breece for his help with the sample preparation and measurements. REFERENCES 1. 2. 3. 4. Goodman, A. M., A Useful Modification of the Technique for Measuring Capacitance as a Function of Voltage, IEEE Trans. Electron Devices ED-21, 753-757 (1974). Goodman, A. M., An Investigation of the Silicon-Sapphire Interface Garton, C. G., Intrinsic and Related Forms of Breakdown in Solids, Goodman, A. M., Semiconductor Measurement Technology: Safe Operation of Capacitance Meters Using High Applied-Bias Voltage, Special Publication 400-34 (December 1976). NBS The apparatus described in the body of this report has been designed for use inside a closed test chamber. This is necessary for electrical and optical shielding of the sample as well as for human safety. This appendix describes a safety-interlock arrangement used in conjunction with the test chamber and provides assembly and construction detail drawings of both the apparatus and the test chamber. A.1.1. Safety Interlock To prevent high (and potentially lethal) voltages from being applied to the sample while the test chamber is open, a pair of microswitches is provided as part of a safety-interlock system. Their location is shown in the Test Chamber Assembly drawing D-1686997 (See section A.1.3 below). The switches, which are closed only when the test chamber cover is fully closed, are wired in series and connected to an insulated BNC A cable from this connector to the high-voltage power supply allows a relay-control circuit in the power supply to sense whether the switch circuit is open or closed. If the switch circuit is closed, voltage may be applied to the sample. If the switch circuit is open, a pair of high-voltage relays inside the power supply is de-energized; this opens the output lines and grounds the output terminals. A.1.2. Probe and Breakdown-Suppression Washer Apparatus The assembly and construction details for this apparatus are shown in drawings C-1687340, C-1687338, C-1687339, C-1687347, C-1687337; a list of the major mechanical parts and drawing reference numbers is given in table 1. A.1.3. Test Chamber The assembly and construction details of the test chamber are shown in drawings C-1687330, C-1687331, C-1687332, C-1687333, C-1687343, D-1686997; a list of the major mechanical parts and drawing reference numbers is given in table 2. Two further explanatory notes: 1. Three high-voltage coaxial chassis connectors are provided in the back of the test chamber. Normally, only two of these are used. The third is provided for possible use with a sample having a guard-ring structure. 2. The poppet valve mounted in the back of the test chamber is connected to the line providing vacuum to the sample base. When the cover is closed, this line is opened to atmospheric pressure relieving the vacuum in the line to the base. This prevents an electrical discharge which might otherwise occur when high voltage is applied to the sample under test. The vacuum hold-down is of course not needed when the sample is held in place by the breakdown-suppression washer. Table 1. List of Major Mechanical Parts Used in |